In an aircraft gas turbine engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against the airfoil section of the turbine blades, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. Thus, there is incentive to raise the combustion gas temperature.
The turbine engine includes compressor disks and turbine disks (sometimes termed “compressor rotors” and “turbine rotors”) and/or turbine shafts and other rotating parts. A number of blades are mounted to the turbine disks/shafts and extend radially outwardly therefrom into the gas flow path. As the maximum operating temperature of the turbine engine increases, the turbine disks/shafts, rotating seal elements, frames, cases, and static seal elements are subjected to higher temperatures. As a result, oxidation and corrosion of the disks/shafts and seal elements have become of greater concern.
Turbine disks/shafts and rotating seals for use at the highest operating temperatures are typically made of nickel and/or cobalt-base superalloys selected for good elevated temperature toughness and fatigue resistance. They have resistance to oxidation and corrosion damage, but that resistance is not sufficient to protect them at the operating temperatures now being reached. Over time, engine deposits, primarily in the form of nickel oxides and/or aluminum oxides, can form a coating or layer on the surface of these turbine components. These engine deposits typically need to be cleaned off or otherwise removed.
However, chemical methods of cleaning turbine engine components to remove engine deposits on the surface thereof often adversely affect or alter the properties of the base metal of the cleaned turbine component, especially when this turbine component comprises a nickel and/or cobalt-containing base metal. Additionally, these chemical cleaning processes also usually have to be repeated several times and/or the chemically treated component requires excessively abrasive mechanical cleaning, for example, by aggressive grit blasting, to provide appropriate clean surface conditions for the turbine component. However, it has been found that excessive chemical cleaning increases the amount of processing time to achieve the desired surface conditions, while aggressive abrasive mechanical cleaning is labor intensive and requires great care to avoid excessive removal of the surface base metal that can alter the desired dimensional geometry of the turbine component.
Additionally, certain chemical compositions that have been used to clean and remove engine deposits from the surface of the turbine component can also excessively etch the surface of the nickel and/or cobalt-containing base metals used in making the turbine component. For example, it has found that chemical etchant compositions comprising acetic acid can cause undesired intergranular attack (i.e., at the grain boundaries) of the nickel and/or cobalt-containing base metal of the turbine component. Such intergranular attack can undesirably weaken the base metal at these grain boundaries.
Accordingly, it would be desirable to be able be able to effectively and efficiently clean and remove engine deposits, especially engine deposits comprising metal oxides, from turbine components that comprise nickel and/or cobalt-containing base metals. It would be especially desirable to be able to clean and remove such engine deposits in a manner that does not excessively or substantially remove or alter the nickel and/or cobalt-containing base metal of the turbine component.